OK, now we're past my experience of the ZyXels. You should access the web interface for the old one, write down the settings for the internet connection, replace it with the new one, and check the new settings match. If not, replace. The browser interface is probably at 192.168.1.1, and if you've never changed the access password, it's probably 1234. Check your manual if in doubt. Many, many, many moons ago, I was an alpha and beta tester for ZyXel. At the time, it was stupidly rugged and over-specced. I have a soft spot for it.